Paint the 'pit' black, and with smallest brush, loosley paint a 'rim' around the edge of the black.

Glue on a couple pieces of fake 'moss'.

I'm not sure you can tell in the photos, but the Karak Stone drybrushing gives it a hint of warmth as well, so it isn't such a stark 'black/white' shade of grey. I'm trying to decide if I'll do a further drybrushing layer, just around the edges of each square, to highlight them just a little bit more, but for now this is how I'm going to do up most of the stone tiles.